In modern medical practice, an important indicator of a patient's condition is the concentrations of gases and ions dissolved in the blood. Traditionally, ionic and gaseous species were measured by removing a sample of blood and assaying it by electrochemical techniques. More recently, direct contact measurements made by sensors placed within the vascular system have been used. Such sensors can be electrochemical in nature or optically based, for example.
In one type of optical blood sensor a matrix material is treated with a sensing component suitable for providing a signal that varies as the concentration of the species of interest varies. Generally, this sensing component is covalently bonded to the matrix material, although this is not always a requirement. This matrix material and sensing component constitute a sensing element. In one such sensor, the sensing component is a fluorescent dye and the matrix material is a cellulose membrane sheet. A small disc is cut from the membrane sheet and is placed in a well of a sensor cassette, which itself is placed in proximity to an optical fiber. An opaque overcoat material can be physically placed over the exposed surface of the disc and secured to the cassette. This overcoat, which is physically separate from the disc, provides optical isolation for the dye in the sensing element. An alternative type of construction includes a disc of cellulose membrane with a fluorescent dye covalently bonded thereto and an overcoat material which is also covalently attached to the disc (see U.S. Pat. No. 5,081,041). In either type of construction, when the sensing component is excited by light imposed on the sensing component, it undergoes fluorescence, emitting a signal. This emission signal is transmitted by the optical fiber to a processor where it is analyzed to provide a determination of the concentration of the species of interest.
Known techniques for preparing such sensors have certain limitations. In some instances, the process requires an inconveniently long time for certain reactions to come to completion. In others, the process requires the use of reagents, or produces intermediates or byproducts, that are toxic. Furthermore, the consistency between sensors which are cut from different parts of the same membrane sheet has been inadequate to provide sensors at the lowest possible cost to the public. Also, certain of the known techniques do not provide adequate sensitivity to the finished device, as a result of low and variable dye concentrations and pKa values.